Electrical make-and-break contacts



May 8, 1934. J. A. `wElGER ELECTRICAL MAKE-AND-BREAK CONTACTSv FiledDeo. 8, 1931 :Snnentor I8u Gftornegs Patented May s, i934 UNITED STATESPATENT foi-*Fica 15 Claims. (CL 14s-11.5)

i This invention relates to electrical make-and break contacts, and moreparticularly to electrical make-and-break contacts of tungsten and amethod of producing the same.

'I'he primary objects of the -invention are the provision of electricalmake-and-break contacts having Agenerally improved characteristics, andvcapable of carryingl and breaking larger currents than` has heretoforebeen possible with the use of contacts common to prior practice whilestill.

maintaining satisfactory service life, have a decreased arcing tendency,give superior perfomance at high and low operating speeds, and haveimproved contact resistance characteristics .during performance.

The invention Afurther contemplates, among other objects, the provisionof electrical makeand-break contacts in which the deleterious effectsusually incident to arcing are pronouncedly reduced, and pitting of thecontact faces, fusion and transfer of material from cooperating contactsare eliminated to a'very substantial degree under the conditions towhich such contacts are ordinarily subjected.

A further object of the invention is the provi- 5 -sion of tungstencontacts andmethod of treating them, wherein and whereby the usual bodystructure is so modified as to have improved working characteristicsthroughout and to present at the contact face a minimum amount of thesubstance .or substances which adversely affect the eflicientfunctioning of the contact.

A still further object of the invention is the provision of suchcontacts and such a method of treating them, wherein and whereby an'interf locking of the grain structure is insured and a reduction `of theimpurities and material in the grain boundary condition to a minimum isobtainedby a treatment with a compound of silicon.

Other objects of the invention will in part be obvious and will in partappear hereinafter.

'I'he invention accordingly comprises the several steps and theVrelation of one or more of such 'steps with respect to each of theothers, and the article possessing the features, propapplications, ofwhich the use in automobile contacts.

erties, and the relation of elements, which are exempled in thefollowing detailed disclosure, and the scope of theapplicationof whichwill be indicated in the claim. 6o

For a fuller understanding of the nature and objects of the inventionreference should be had to the followingdetailed description taken inconnection with the accompanying drawing, in which:

, Fig. 1 illustrates a portion of the contact surface of a typicalcontact such as hadrheretofore been used, magnified 1000 diameters;

Fig. 2 illustrates a portion of a contact surface of a tungsten contactmaterial in which the grains are interlocked, magnified 1,000 diameters;

Fig. 3 illustrates a portion of the contact surface of a tungstencontact prepared from the material depicted in Fig. 2, heat treated toob- 7.. tain a large grain4 structure, magnified 29 .dio ameters; and

Fig. 4 illustrates a portion of the contact surface of a tungstencontact which has been-treated in accordance with the present inventionto obtain an interlocked grain structure, magnified 29 diameters.

'Heretofore electrical make-and-break contacts of tungsten have beenextensively used in various ignition systems may be taken asrepresentative. Such contacts up to the present time have beencustomarily made of wrought tungsten, either by sawing contact discs'from a wrought tungsten rod, or by punching them from a rolled tungstensheet.

Such contacts develop a number of faults in service. For example, therepeated breaking of current which is required results in periodicarcing, attended with a transfer of metal from one cooperating contactto the other, with consequent pitting of one contact face and buildingup of projections on the other. Also, the continued4 transfer ofmaterial from one contact to another often brings about a localizationof the contact area, with a resultant increase in current density atsuch restricted area of contact. This condition, in the case of priorcontacts, renders more pronounced the deteriorating action of the arc.The transfer of material under these circumstances sometimes becomes sogreat that a sucient amount thereof is built up on one ofthemake-and-break contacts to prevent Y the breaking of the circuit byseparation of the Such factors of deterioration bring 110 and the factthat an increase in the amount of current broken at their faces hastenstheir deterioration. Consequently the permissible current densities andamperages which have heretofore been usable in ignition and similarmake-and-break systems have been restrictively limited to an amountcommensurate with satisfactory service life of the contacts. That is,with contacts of the typel hitherto in general use, unless the contactsurface was comparatively large, and the amperage comparatively low, theservice life of the contacts was unduly shortened.

A proper understanding of these shortcomings in prior contacts, and ofthe nature of the present invention whereby such unsatisfactorycharacteristics are eliminated or greatly mitigated, calls forconsideration of the microstructure of the contact metal. To study themicrostructure,

' the contact surface is polished and then lightly etched with asuitable chemical reagent which serves to develop the grain structureand the boundaries between the grains.

The drawings forming a part of this application are representations ofthe various structures, shown under the microscope.

Microscopic examination of the surface of the best prior art tungstencontacts, as illustrated in Fig. l, shows the presence of very finegrains, in the neighborhood of 20,000 per square millimeter, each ofwhich is surrounded and separated from adjacent grains by materialcharacv terized by properties different from those of the main body ofthe metallic grains. Throughout this specication and in the accompanyingclaims, such material will 'be referred to as material in the grainboundary condition, Whether it is produced in the course of the grainstructure formation and actually denes the surfaces of the severalgrains, or is produced in the working of the contact metal, as byswaging, rolling, grind- .boundary condition) exposed on theY workingsurface of such contacts is large in proportion to the surface area ofthe contacts, and in addition the strain-hardening of the ,material ofthe grains l0 may contribute to the amount of material in the grainboundary condition.

'Y It has been found that the material in the grain boundary conditionis much more active physically and chemically than the material at thecenter of the grains. Whether this be due to greater concentration ofimpurities at the grain boundaries or to amorphous condition of thegrain boundary material, or to some other cause,

is not as yet positively established. But whatever may be its precisenature, this material in the grain boundary condition has propertiesdifferent from those of the grains themselves.

Many observations .of make-and-break contacts of tungsten of the typesheretofore generally used, have indicated that the eiects ofdeterioration in actual service manifest themselves rst and moremarkedly 'at the grain boundaries separating the individual lgrains inthe Working surface of the contact. It appears that the material in thegrain boundary condition has lower resistance to the deterioratinginuences imposed under service conditions than has the materialcomprising the grain proper. It Volatilizes more readily than thematerial of the grains themselves, and these facts together with theother properties peculiar to it, appear to be largely responsible forthe arc deterioration and other faulty characteristics of contactsheretofore used.

A structure in which the grains are equiaxed, a condition depicted inFig. 3, has been found to increase the likelihood of poor operatingcharacteristics. that, since the electric arc between two such contactsgradually etches away impurities and materialin the grain boundarycondition, the grains become loosened and eventually transfer to theopposite contact, resulting in the pittingof one contact and a buildingup on the other contact.

In the application of Childress B. Gwyn, Serial No. 551,671, led July18, 1931 and assigned to P. R. Mallory & Co., Incorporated, methods aredisclosed which reduce the material in the grain boundary condition,thus considerably enhancing the operating characteristics of contacts sotreated. In accordance with the present invention, it has been foundthat their operating characteristics can be further improved by insuringan interlocked grain structure and by treating This is believed to bedue to the fact the contact material with an ingredient or in- Ygredients which stimulate and accelerate grain growth, and reduce to aminimum impurities and material in the grain boundary condition.

In accordance with the presentinvention, a greatly improved contactmaterial may be obtained by increasing the size of the grains to a sizeof less than 2,000 per square millimeter'by heat treatment, care -beingtaken to regulate the time and temperature to grow the grains withoutequiaxing them to insure an interlocked grain structure. The equiaxingtemperature for tungsten is about 2400o C., for a short duration.

The operating characteristics of such a structure can be furtherimproved in accordance with the present invention by treating metal fromwhich the contacts are' to be formed, in such a manner as to reduce theimpurities and material in the ygrain boundary condition. Such areduction is performed in the preferred form by treating the materialfrom which the contacts are to be formed with an ingredient whichreduces such impurities and which can then be volatilized to leave themetal in a substantially pure form. It hasbeen found that a compound ofsilicon, such as, for example, sodium silicate, which can be reducedto.silicon oxide by heat treatment, will not only perform these desiredfunctions, but -will, in addition, stimulate and accelerategrain-growth. y

Such a method may bespracticed in the following manner, although it isto be understood that the present invention is not to be limitedthereto, in regard to the number of steps taken, their order, and thespecific values set forth in each step. in the treatment, since it isobvious that the invention may be practiced by other specific methods,or the desired features of the V'tungsten contacts may be obtainedv invarying degrees by the practice of methods of this same generalcharacter, differing from the preferred method chiefly in specic valuesof the treatments constituting the different steps of the method, or the.order in which they are taken. A The preferred method is practiced byaddin to pure tungsten in the oxide form a small amount of siliconoxide, approximately .5 per cent in the form of sodium silicate. 'I'hemixture is thoroughly mixed to insure an even distribution of thesilicon compound throughout the mass of the oxide of tungsten. Thismixture is then fired, preferably in a silica crucible, at approximately1100 C., for about one hour,- to agglomerate the finely dividedparticles into coarser particles and to convert the sodium silicate intosilicon oxide. 'Ihe fired mixture is then reduced by heating in areducing atmosphere, such as, for example, hydrogen, at a temperatureincreasing from about 400 C., to about 900 C., for a considerable periodof time, such as, for about two hours. The mixture is then removed fromthe reduction furnace and passed through a sieve of about 200 mesh,after which it is pressed into slugs by considerable pressure,approximating about ten tons per square inch.

The pressed slug is then baked in a non-oxidizing atmosphere at atemperature of about 1250 C., for a short period of time, preferablyabout fifteen minutes, and is then heat treated -in a non-oxidizingatmosphere, preferably hydrogen, by passing an electric current throughit, the current being increased gradually until the slug is heated toabout between 3000 C., and 3200 C., the heat being maintained at themaximum temperature for a short period of time, such as, for about tenminutes, after which the ow of current is discontinued and the slugallowed to cool rapidly.

During this heat treatment, the silicon com- 1 pound reduces theimpurities, including oxygen,

in the tungsten, decreases the amount of material in the grain boundarycondition and in addition, this ingredient accelerates and stimulatesthe grain/ growth, thereby producing a material with a..l relativelylarge grain structure. After performing its desired functions thisingredient is vlargely volatilized by this heat treatment.

The treated slugs are then hot-swaged into rods, the temperature beingaccurately controlled to avoid equiaxing the grain structure of themetal and to insure the production of a strain worked metal withdistorted and interlocked L grains, a condition depicted in Fig. 2. Theswaged rods are then heated in a non-oxidizing atmosphere, preferably ofhydrogen, by passing an electric current through them, the metal beingquickly heated to a temperature of about 2400 C., the temperature beingheld at the maximumvalue for a short period of time, approximaterly twominutes, after which the metal is cooled rapidly. This latter heattreatment is performed to obtain an increase in the size of the grains,the temperature being increased to the maximum Value depicts suchamaterial having a grain count of about per square millimeter.Advantageous results are obtained through the use of electricalmake-and-break contact material having a grain count much higher thanthat of the specic material depicted, produced by the practice of theabove outlined method, the upper limit being about 2,000 per squaremillimeter. Since the last step in the method outlined above is chieflythe determining factor in this respect, any desired grain size withinthe limits set forth may be obtained by proper control of this heattreatment.

Comparative tests were conducted to determine the relative merits ofVvarious .types of contacts, including those made in accordance with thepresent invention. These contacts were operated at 8.75 volts across thecontacts and approximately 6.25 break amperes, making and breaking thecircuit 2,200 times per minute, and showed the following results after61 hours of operation:

y 1. Standard fine grain tungsten contacts of 20,000 grains per squaremillimeter, showed 16 per cent failures;

2. Large grain equiaxed tungsten contacts with no reduction in thematerial in the grain boundary condition showed 33 per cent failurevs;and

3. Distorted interlocked grain tungsten contacts, retreated to reducethe material in the grain boundary condition, showed no failures.

Electrical make-and-break contacts, made in accordance with the presentinvention, efficiently attain the objects set forth above in a veryadmirable manner, improved service life and operation at much highercurrent densities being apparently attributable to the facts that sincethe material in the grain boundary condition has been materiallyreduced, very limited areas of this material' are subjected to theaction of the arc, the result being that the arc will wander moreuniformly over the entire surface of the contact, Vinstead ofconcentrating on one local spot; and even after the wearing away of thematerial in the grain boundary condition, the grains are securely heldin their positions by the mechanical interlock. i

Electrical make-and-break contacts, made in accordance with the presentinvention, can be further improved by further treatment of the contactsurfaces thereof to additionally reduce the material in the grainboundary condition. 125 Such a treatment has been disclosed in theapplication of Childress B. Gwyn, Serial No. 551,671, mentioned above.

Since certain changes may be made in the contacts and in the process ofmanufacture described 130 above without departing from the scope oftheinvention, it iswintended that all matter contained in the abovedescription or shown in the accompanying drawing shall be interpreted asillustrative and not in a limiting sensei yIt is also to be understoodthat the following claims are intended to cover all of the generic andspecific featuresof the invention herein described and all statements ofthe scope of the invention which as a matter' of language might be saidto fall therebetween.

Having described my invention,` what I claim as new and desire to secureby LettersPatent, is:

1. An electrical make-and-break contact of tungsten having aninterlocked grain structure 145 of relatively large grains and theimpurities and material in the grain boundary condition reduced 'to aminimum by the volatilization by heat of a previously incorporated Watersoluble alkali metal silicate which stimulates graingrowth. w

2. An electrical make-and-break contact of tungsten having aninterlocked grain structure of relatively large grains and theimpurities and material in the grain boundary condition reduced to aminimum by the volatilization by heat of previously incorporated siliconoxide formed by heat conversion of a water soluble alkali metal silicatewhich stimulates grain growth.

3. An electrical make-and-break contact of tungsten having aninterlocked grain structure and the impurities and material in the grainboundary condition reduced to a minimum.

4. An electrical make-and-break contact of tungsten having a structureof relatively large interlocked grains in which the impurities andmaterial in the grain boundary condition have been reduced to a minimum.

5. An electrical make-and-break contact having a tungsten face of aninterlocked grain structure substantially free from material in thegrain boundary condition.

6. An electrical make-and-break contact having a tungsten face of aninterlocked grain structure with a grain count of less than 2000 persquare millimeter and being substantially free from material in thegrain boundary condition.

'7. The method of producing an electrical makeand-break contact materialcomprising mixing, with an oxide of tungsten a water soluble alkalimetal silicate, heat treating the mixture to convert the silicate to anoxide of silicon and to reduce the tungsten oxide to tungsten,heat-treating at a temperature of between about 88 per cent and 95 percent of the melting point of tungsten to cause the silicon oxide toperform a grain growth accelerating action, remove impurities andmaterial'in the grain boundary condition and to volatilize the siliconoxide, and mechanically 1 working and heat-treating the tungsten at atemperature above the equiaxing temperature of. tungsten to obtain abody in which the grains are of a desired size and interlocked, thetemperatures during the latter heat treatment' being rapidly carriedthrough the equiaxing temperature of tungsten.

8. The method of producing an electrical makeand-break contact materialcomprising mixing with an oxide of tungsten a water soluble'alkali metalsilicate, firing the mixture to convert the silicate to an oxide ofsilicon, reducingthe mixture in a reducing atmosphere with heat tochange the` tungsten oxide to tungsten, pressing the resultant mixtureinto a slug, heat-treating the slug at a temperature between aboutv3000" C. and 3200 C. to cause the silicon oxide to perform a graingrowth accelerating action and remove impurities in the material in thegrain boundary condition, maintaining the heat at a maximumtemperaturegto volatilize the silicon oxide and hot swaging the slug oftungsten into' a rod ofv strain workedcontact material vata'temperatureE which insures an interlocked grain structure.

9. The method of producing an electrical makeand-break contact materialcomprising mixing with an oxide of tungsten a waterv soluble alkalimetal silicate, converting the silicate -to` silicon oxide by heatreducing the mixture in a reducing atmosphere with heat to change thetungsten oxide tov tungsten, pressing the resultant mixture into a slug,heat treating the slug at a temperature between about 3000? C. and 3200C. to cause the silicon oxide to perform a grain growth acceleratingaction and remove impurities and material in the grain boundarycondition, maintaining the heat at a maximum to volatilize the siliconoxide, hot swaging the slug of tungsten into a rod of strain workedcontact material at a temperature which insures an interlocked grainstructure, and heat treating the rod-in a non-oxidizing atmosphere at atemperature which insures a desired grain growth after passing quicklythrough the equiaxing temperature to prevent destruction of theinterlocked grain structure.

10. 'I'he method of producing an electrical make-and-break contact'material comprising mixing tungsten oxide with sodium silicate, treatingwith heat to convert the sodium silicate to silicon oxide,.reducing the.mixture in a reducing atmosphere with heat to reduce the tungsten oxideto tungsten, pressing the resultant mixture'into a slug,` heat treatingthe slug at a temperature between about 3000 C. and 3200 C. to cause thesilicon oxide to perform a grain growth accelerating action and removethe impurities and material in the grain boundary condition, maintainingthe heat at a maximum temperature to volatilize the silicon oxide andhot swaging the slug of tungsten into a rod of strain worked contactmaterial at a temperature which insures an interlocked grain structure.

l1. In a method of producing make-and-break contact material, the stepcomprising heating at a temperature between aboutk300o C. and 3200 C. toaccelerate and stimulate the grain growth and to remove impurities andvmaterial in the grain boundary condition a mixture of tungsten oxideand an oxide of silicon formed by heat conversion -of a water solublealkali metal silicate.

12. In a method of producing an electrical make-and-break contactmaterial, the steps comprising heating at a temperature between about3000 C. and 3200 C. to accelerate and stimulate grain growth and toremove impurities and the materialin the grain boundary condition amixture of tungsten oxide and an oxide of silicon formed by heatconversion of a water soluble4 -grains of relatively large size obtainedby heating to a temperature above the equiaxing temperature, equiaxingbeing avoided by passing rapidly through the equiaxing temperature.

14. The method of producing an electrical make-and-break contactmaterial comprising mixing tungsten oxide and a water soluble alkalimetal silicate, ring the mixture to convert the silicate to an oxide ofsilicon, heating theresultant mixture of tungsten oxide and siliconoxide in a reducing atmosphere at a temperature between about 800 C. and1000 C. to reduce the tungsten oxide to tungsten, ,forming a slug fromthe mixture, heating the slug at a temperature Vbetween about 88 percent and 95 .per cent ofthe fusing temperature lof tungsten tovolatilizezthe y silicon oxide and cause it to decrease the amount ofmaterial i'nthe grain boundary condition and accelerate and stimulategrain growth, hot swaging the slug to form a strain worked rod having ftemperature to promote grain growth without equiaxing the grains.

15. 'I'he method of producing `an electrical make-and-break contactmaterial comprising mixing tungsten oxide with an oxide of silicon inthe form of a water soluble alkali metal silicate, iiring the mixture ata temperature of about 1100 -C. for a time suicient to convert thesilicate to silicon oxide, ring the mixture in a reducing atmosphere ata temperature of about between 800 C. and 1000 C. for a time suicient toreduce the tungsten oxide to tungsten, pressing the mixture into a slug,heating the slug in a non-oxidizing atmosphere at a temperature betweenabout 3000 C. and 3200" C. for a short period of time to volatilize thesilicon oxide and 4cause it to decrease the amount of material in thegrain boundary condition and accelerate and stimulate grain growth, hotswaging the slug into a rod of strain worked metal with distorted andinterlocked grains, quickly heating the strain' worked rod to atemperature higher than the equiaxing temperature to increase the sizeof the grains and rapidly cooling the rod.

JOSEPH A. WEIGER.

